Magnetic discs with magnetizable media are used for data storage in most all computer systems. Current magnetic hard disc drives operate with the read-write heads only a few nanometers above the disc surface and at rather high speeds, typically a few meters per second.
Generally, the discs are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
FIG. 1 shows a schematic of a magnetic disc drive for which a spindle motor having a fluid dynamic bearing manufactured by the method and apparatus of the present invention is particularly useful. Referring to FIG. 1, a disc drive 100 typically includes a housing 105 having a base 110 sealed to a cover 115 by a seal 120. The disc drive 100 has a spindle 130 to which are attached a number of discs 135 having surfaces 140 covered with a magnetic media (not shown) for magnetically storing information. A spindle motor (not shown in this figure) rotates the discs 135 past read/write heads 145 which are suspended above surfaces 140 of the discs by a suspension arm assembly 150. In operation, spindle motor rotates the discs 135 at high speed past the read/write heads 145 while the suspension arm assembly 150 moves and positions the read/write heads over one of a several radially spaced tracks (not shown). This allows the read/write heads 145 to read and write magnetically encoded information to the magnetic media on the surfaces 140 of the discs 135 at selected locations.
Over the years, storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the read/write heads must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the read/write heads. This is known as a “crash” and can damage the read/write heads and surface of the storage disc resulting in loss of data.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of considerable importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping.
One alternative bearing design is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. In addition to air, typical lubricants include oil or other fluids. Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat run out.
Dynamic pressure-generating grooves (i.e., hydrodynamic grooves) disposed on journals, thrust, and conical hydrodynamic bearings generate localized area of high fluid pressure and provide a transport mechanism for fluid or air to more evenly distribute fluid pressure within the bearing, and between the rotating surfaces. The shape of the hydrodynamic grooves is dependant on the pressure uniformity desired. The quality of the fluid displacement and therefore the pressure uniformity is generally dependant upon the groove depth and dimensional uniformity. For example, a hydrodynamic groove having a non-uniform depth may lead to pressure differentials and subsequent premature hydrodynamic bearing or journal failure.
As the result of the above problems, electrochemical machining (ECM) of grooves in a hydrodynamic bearing has been developed. Broadly described, ECM is a process of removing material metal without the use of mechanical or thermal energy. Basically, electrical energy is combined with a chemical to form an etching reaction to remove material from the hydrodynamic bearing to form hydrodynamic grooves thereon. To carry out the method, direct current is passed between the workpiece (e.g., counter plate, sleeve journal, or a conical bearing) which serves as an anode and the electrode, which typically carries the pattern to be formed and serves as the cathode, the current being passed through a conductive electrolyte which is between the two surfaces. At the anode surface, electrons are removed by current flow, and the metallic bonds of the molecular structure at the surface are broken. These atoms go into solution, with the electrolyte as metal ions and form metallic hydroxides. These metallic hydroxide (MOH) molecules are carried away to be filtered out. However, this process raises the need to accurately and simultaneously place grooves on a surface across a gap which must be very accurately measured, as the setting of the gap could determine the rate and volume at which the metal ions are carried away. Even in simple structures, this problem can be difficult to solve. When the structure is the interior surface of a conical bearing, the setting of the gap width can be extremely difficult. Manufacturability issues associated with conical parts often make it difficult to control the diameter of the cones. Due to mechanical tolerances, the workpiece may be misaligned with the electrode causing an uneven gap and a correspondingly uneven depth hydrodynamic groove. Therefore, it is difficult to make a tool with fixed electrodes that could guarantee a continued consistent workpiece to electrode gap to form dimensionally consistent hydrodynamic grooves.
Advanced groove patterns on thrust and journal are currently manufactured by an ECM process. The electrode used in the ECM process is made of high conductivity material and usually has a cylindrical shape with workpiece surface machined to reflect 3D pattern of a particular shape and depths.
The ECM process uses a shaped electrode to supply electrical flux fields thru an electrolyte to cause metal removal from the work piece in the areas influenced by these fields. The electrode has regions of conducting material separated by regions of insulating material. The shape and pattern of these regions is generally in the reverse image of the areas to be machined by the electrochemical action. Machining occurs in the zones of the conducting region and is restricted in the zones of insulating material. These electrodes may be complex and multidimensional in shape.
Fabrication of these electrodes is dependent on techniques that allow construction of alternating zones of conducting and insulating materials. Typical restrictions in fabrication capability include limited sizes of traditional machine cutting tools and capability to form single piece multidimensional structures. We propose photolithography as a possible cost saving technique since we can utilize batch processes in deposition and etch.
The electrode workpiece surface pattern can be manufactured by a variety of techniques such as milling, etching or laser machining. Once the 3D pattern is manufactured on the electrode workpiece surface the dents are filled with the dielectric material followed by the dielectric grinding step bringing the dielectric layer electrode workpiece zone (metallic surface) to the same level as the dielectric surface. The ECM electrode manufactured this way is schematically shown in FIG. 2. The layer of dielectric acts as the separator of the electric field between two neighboring active zones on the electrode and generally determines the feature width of the workpiece based on the feature width of the electrode workpiece surface pattern. The disadvantages of such electrode design are the following:    (a) An increased probability of arcing with a reduction in machining gap;    (b) An occasional mechanical contact of the electrode workpiece surface with the sample surface that leads to damaging the electrode workpiece surface and its destruction; this is especially characteristic in mass production;    (c) If the conductive path is narrow the metal/dielectric interface is prompt to overheating that leads to electrode disintegration;    (d) The smallest feature width that can be obtained by conventional milling technique for this electrode design is 35 microns;    (e) The laser machining time for feature pattern is of the order of several hours per electrode; and    (f) The throughput limit is 1 electrode per system.
Therefore, a need exists for improved electrodes and method of manufacturing the same to provide a reliable method and apparatus for forming hydrodynamic grooves that is accurate and cost effective.